214 
Proceedings of Royal Society of Edinburgh. 
of particles on the mornings of the 10th and 11th April — hut the 
air on the 10th was very clear, owing to it being very dry, the wet 
bulb being depressed 13°; while the morning of the 11th was thick, 
owing to a great increase in the humidity, the wet bulb on this 
occasion being depressed only 2°. 
The Bellagio and Baveno observations show the same result, but 
not so satisfactorily. Though every precaution was taken to test the 
air where it was free from local pollution, yet it is evident this was 
not possible under the conditions at these stations. 
Coming now to the Rigi observations, it will be noticed that the 
air was thickest on the 21st, when it contained the fewest particles. 
This thickness was owing to humidity, a good deal of moisture being 
deposited on the dust. It will be noticed that there was a depres- 
sion of 1°*5 in the wet bulb at the time, showing that the air 
was not saturated. As this was the first occasion on which I had 
made observations on this dense form of condensation, I carefully 
noted the state of the air with regard to dryness. The depression 
of the wet bulb given in the table may not be quite correct, though 
taken as carefully as possible under the conditions. It may possibly 
be too great, owing to the air having been heated locally. It was 
observed that all objects in the open air were quite dry, not only 
stones which might have been kept dry by heat communicated from 
beneath, but all surfaces, such as wooden fences, &c. The only 
exception to this was the grass, which carried drops of moisture at 
the tips of its blades, but this came from within the plant, there 
being no indication of the other parts of the blades being wetted 
by fog. 
It is, however, very evident that though all exposed surfaces were 
dry on this occasion, it does not necessarily follow that the air 
was not saturated. The reason why the fog particles did not 
wet the exposed surfaces may have been that these surfaces were 
all heated by radiation, and the fog particles would be evaporated 
in the hot air in contact with the exposed surfaces, as radiation can 
undoubtedly penetrate a great depth of fog. I regret that no 
observations were made in this direction.* It may be noticed in 
passing, that the Swiss Meteorological Report for the Rigi Kuhn 
shows the depression of the wet bulb at one o’clock on the 25th 
* It has, however, been confirmed by later observations. 
